1. Field of the Invention
The present invention relates generally to a communication system utilizing an Orthogonal Frequency Division Multiplexing (‘OFDM’) scheme, and in particular, to an apparatus and method for dynamically assigning resources using channel quality information that is separately fed back on subbands.
2. Description of the Related Art
Since the late 1990's, South Korea has partially deployed an IMT-2000 (International Mobile Telecommunication-2000) system, a 3rd generation (3G) mobile communication system, aimed at advancing wireless multimedia service, global roaming, and high-speed data service. The 3G mobile communication system was developed especially to transmit data at high rate in compliance with the increase in amount of service data. Accordingly, the 3G mobile communication system has evolved into a packet service communication system that transmits burst packet data to a plurality of mobile stations and is designed for transmitting mass data.
Consequently, the packet service communication system is being developed for a high-speed packet service. For example, High Speed Downlink Packet Access (HSDPA), which is under standardization in 3rd Generation Partnership Project (3GPP), a standardization organization for the asynchronous 3G mobile communication system, has recently introduced Adaptive Modulation and Coding (AMC) scheme, Hybrid Automatic Retransmission Request (HARQ) scheme, and Fast Cell Select (FCS) scheme in order to support a high-speed packet data service.
The AMC scheme refers to a data transmission scheme for adaptively determining different channel modulation schemes and coding schemes according to a channel condition between a cell (or a base station (BS)) and a mobile station (MS), thereby improving efficiency of the entire cell. The AMC scheme has a plurality of modulation schemes and a plurality of coding schemes, and modulates and codes a channel signal using a preferred combination of the modulation schemes and the coding schemes. Commonly, each combination of the modulation schemes and the coding schemes is called a Modulation and Coding Scheme (MCS), and a plurality of MCSs with a level 1 to a level N can be defined according to the number of MCSs. That is, the AMC scheme adaptively determines a level of the MCS according to a channel condition between the mobile station and the base station to which the mobile station is currently wirelessly connected, thereby improving efficiency of the entire base station system. The AMC scheme, HARQ scheme, and FCS scheme can be used not only in the HSDPA scheme but also in all other scheme s for high-speed data transmission.
Currently, the 3G mobile communication system is developing into a 4th generation (4G) mobile communication system. The 4G mobile communication system is under standardization, aimed at efficient interworking and an integrated service between a wired communicant network and a wireless communication network, beyond a simple wireless communication service provided in the earlier-generation mobile communication system. Therefore, many studies and experiments are being conducted on scheme s for transmitting a large volume of data (e.g., approaching the capacity of a wired communication network) in a wireless communication network. Further, in the 4G mobile communication system, active research is being performed on a Dynamic Channel Allocation (DCA) scheme for dynamically assigning channels based on an individual channel condition of each mobile station in order to transmit mass data.
Accordingly, in the 4G mobile communication system, active studies are being made of an OFDM scheme as useful scheme for high-speed data transmission in wired/wireless channels. The OFDM scheme, a scheme for transmitting data using multiple carriers, is a kind of Multi-Carrier Modulation (MCM) for parallel-converting a serial input symbol stream and modulating the parallel-converted symbols with a plurality of orthogonal subcarriers before transmission.
The OFDM scheme, although it is similar to a conventional Frequency Division Multiplexing (FDM) scheme, is characterized in that it can secure optimal transmission efficiency during high-speed data transmission by maintaining orthogonality between subcarriers. In addition, the OFDM scheme is characterized in that it has high frequency efficiency and is robust against multipath fading, thereby securing optimal transmission efficiency during high-speed data transmission. Further, because the OFDM scheme uses overlapped frequency spectrums, it has high frequency efficiency, is robust against frequency selective fading and multipath fading, reduces Inter-Symbol Interference (ISI) using a guard interval, enables design of an equalizer with a simple hardware structure, and is robust against impulse noises. For such advantages, the OFDM scheme shows a tendency to be actively applied to communication systems.
FIG. 1 is a block diagram schematically illustrating a structure of a conventional communication system utilizing the OFDM scheme (OFDM communication system). Referring to FIG. 1, the OFDM communication system includes a transmitter, for example, a base station transmitter 100, and a receiver, for example, a mobile station receiver 150.
The base station transmitter 100 includes a Cyclic Redundancy Check (CRC) inserter 111, an encoder 113, a resource assignment controller 115, a symbol mapper 117, a channel multiplexer (MUX) 119, a serial-to-parallel (S/P) converter 121, a pilot symbol inserter 123, an inverse fast Fourier transform (IFFT) unit 125, a parallel-to-serial (P/S) converter 127, a guard interval inserter 129, a digital-to-analog (D/A) converter 131, and a radio frequency (RF) processor 133.
When there are user data bits and control data bits to transmit, the user data bits and the control data bits are input to the CRC inserter 111. Herein, the user data bits and the control data bits will be referred to as information data bits, and the control data includes resource assignment information applied in the resource assignment controller 115, i.e., AMCS (Adaptive Modulation and Coding Scheme) information (or MCS level information), channel multiplexing information, and transmission power information. The CRC inserter 111 inserts CRC bits in the information data bits, and outputs CRC-inserted information data bits to the encoder 113. The encoder 113 encodes the signal output from the CRC inserter 111 using a predetermined coding scheme received from the resource assignment controller 115, and outputs the encoded signal to the symbol mapper 117. Turbo coding scheme or convolutional coding scheme having a predetermined coding rate can be used as the coding scheme. The resource assignment controller 115 can control either or both of the coding rate and the coding scheme according to conditions of the OFDM communication system. The resource assignment controller 115 determines channel conditions between the base station and a mobile station based on Channel Quality Information (CQI) fed back from a mobile station transmitter (not illustrated in FIG. 1). For example, the CQI can be a Signal-to-Noise Ratio (SNR).
The symbol mapper 117 modulates the coded bits output from the encoder 113 into modulation symbols using a corresponding modulation scheme under the control of the resource assignment controller 115, and outputs the modulation symbols to the channel multiplexer 119. For example, quadrature phase shift keying (QPSK) scheme or 16-ary quadrature amplitude modulation (16QAM) scheme can be used as the modulation scheme. The channel multiplexer 119 channel-multiplexes the modulation symbols output from the symbol mapper 117 under the control of the resource assignment controller 115, and outputs the channel-multiplexed symbols to the serial-to-parallel converter 121. The resource assignment controller 115 controls the channel multiplexer 119 such that among the subchannels available in the OFDM communication system, an optimal subchannel is assigned to a corresponding mobile station according to channel conditions between the base station and the mobile station. That is, the resource assignment controller 115 controls the channel multiplexer 119 such that among the subchannels available in the OFDM communication system, a subchannel capable of maximizing the entire frequency efficiency, when it is assigned to a corresponding mobile station, should be assigned to the corresponding mobile station. Herein, the subchannel refers to a channel including at least one subcarrier. In the following description, it will be assumed that each subchannel includes one subcarrier, for the convenience of explanation. Further, in the following description, the subchannel and the channel have the same meaning.
The channel multiplexer 119 dynamically multiplexes channels on a dynamic channel assignment basis according to channel conditions between the base station and the mobile station, thereby improving system performance.
If a channel condition between the base station and the mobile station is relatively excellent, the resource assignment controller 115 changes a current modulation scheme to a new modulation scheme having a higher order than an order of the current modulation scheme, and changes a current coding scheme to a new coding scheme having a higher coding rate than a coding rate of the current coding scheme. No matter how excellent the channel condition is, if the current modulation scheme has the highest possible order, the resource assignment controller 115 maintains the current modulation scheme, and if the current coding rate is the highest possible coding rate, the resource assignment controller 115 maintains the current coding rate.
However, if a channel condition between the base station and the mobile station is relatively poor, the resource assignment controller 115 changes a current modulation scheme to a new modulation scheme having a lower order than an order of the current modulation scheme, and changes a current coding scheme to a new coding scheme having a lower coding rate than a coding rate of the current coding scheme. No matter how poor the channel condition is, if the current modulation scheme has the lowest possible order, the resource assignment controller 115 maintains the current modulation scheme, and if the current coding rate is the lowest possible coding rate, the resource assignment controller 115 maintains the current coding rate.
In addition, the resource assignment controller 115 controls the channel multiplexer 119 such that among the channels available in the base station, a channel capable of providing the best channel condition, when it is assigned to a corresponding mobile station, should be assigned to the corresponding mobile station, thereby improving the entire system performance. Although not illustrated in FIG. 1, the resource assignment controller 115 also controls transmission power to be applied to a channel assigned to the corresponding mobile station by the channel multiplexer 119. An operation of assigning a channel, and determining an MCS level and transmission power by the resource assignment controller 115 will be described in more detail herein below.
The serial-to-parallel converter 121 parallel-converts the channel-multiplexed serial modulation symbols output from the channel multiplexer 119, and outputs the parallel-converted modulation symbols to the pilot symbol inserter 123. The pilot symbol inserter 123 inserts pilot symbols into the parallel-converted modulation symbols output from the serial-to-parallel converter 121, and outputs the pilot-inserted modulation symbols to the IFFT unit 125.
The IFFT unit 125 performs N-point IFFT on the pilot-inserted modulation symbols output from the pilot symbol inserter 123, and outputs the IFFT-processed modulation symbols to the parallel-to-serial converter 127. The parallel-to-serial converter 127 serial-converts the IFFT-processed parallel modulation symbols output from the IFFT unit 125, and outputs the serial-converted modulation symbols to the guard interval inserter 129. The guard interval inserter 129 inserts a guard interval signal into the serial-converted modulation symbols output from the parallel-to-serial converter 127, and outputs the guard interval-inserted modulation symbols to the digital-to-analog converter 131. The guard interval is inserted to remove interference between a previous OFDM symbol transmitted at a previous OFDM symbol time and a current OFDM symbol to be transmitted at a current OFDM symbol time in the OFDM communication system. The guard interval is inserted in a cyclic prefix scheme or a cyclic prefix scheme. In the cyclic prefix scheme, a predetermined number of last samples of an OFDM symbol in a time domain are copied and inserted into a valid OFDM symbol, and in the cyclic postfix scheme, a predetermined number of first samples of an OFDM symbol in a time domain are copied and inserted into a valid OFDM symbol.
The digital-to-analog converter 131 analog-converts the signal output from the guard interval inserter 129, and outputs the analog-converted signal to the RF processor 133. The RF processor 133, including a filter and a front-end unit, RF-processes the signal output from the digital-to-analog converter 131, and transmits the RF-processed signal via a transmission antenna.
The mobile station receiver 150 includes an RF processor 151, an analog-to-digital (A/D) converter 153, a guard interval remover 155, a serial-to-parallel (S/P) converter 157, a fast Fourier transform (FFT) unit 159, an equalizer 161, a pilot symbol extractor 163, a channel estimator 165, a parallel-to-serial (P/S) converter 167, a channel demultiplexer (DEMUX) 169, a resource assignment controller 171, a symbol demapper 173, a decoder 175, and a CRC remover 177.
The signals transmitted by the base station transmitter 100 are received via a reception antenna of the mobile station receiver 150. The received signals experience a multipath channel and have a noise component. The signals received via the reception antenna are input to the RF processor 151. The RF processor 151 down-converts the signals received via the reception antenna into an intermediate frequency (IF) signal, and outputs the IF signal to the analog-to-digital converter 153. The analog-to-digital converter 153 digital-converts an analog signal output from the RF processor 151, and outputs the digital-converted signal to the guard interval remover 155. The guard interval remover 155 removes a guard interval signal from the digital-converted signal output from the analog-to-digital converter 153, and outputs the guard interval-removed signal to the serial-to-parallel converter 157. The serial-to-parallel converter 157 parallel-converts the serial signal output from the guard interval remover 155, and outputs the parallel-converted signal to the FFT unit 159.
The FFT unit 159 performs N-point FFT on the signal output from the serial-to-parallel converter 157, and outputs the FFT-processed signal to the equalizer 161 and the pilot symbol extractor 163. The equalizer 161 channel-equalizes the signal output from the FFT unit 159, and outputs the channel-equalized signal to the parallel-to-serial converter 167. The parallel-to-serial converter 167 serial-converts the parallel signal output from the equalizer 161, and outputs the serial-converted signal to the channel demultiplexer 169. The channel demultiplexer 169 channel-demultiplexes the serial-converted signal output from the parallel-to-serial converter 167 under the control of the resource assignment controller 171, and outputs the channel-demultiplexed signal to the symbol demapper 173. The resource assignment controller 171 controls a channel demultiplexing operation of the channel demultiplexer 169 based on channel multiplexing information in the control data transmitted from the base station transmitter 100.
The FFT-processed signal output from the FFT scheme 159 is input to the pilot symbol extractor 163. The pilot symbol extractor 163 extracts pilot symbols from the FFT-processed signal output from the FFT scheme 159, and outputs the extracted pilot symbols to the channel estimator 165. The channel estimator 165 performs channel estimation on the extracted pilot symbols output from the pilot symbol extractor 163, and outputs the channel estimation result to the equalizer 161. The channel estimator 165 performs a channel estimation operation on each of the subcarriers. The mobile station receiver 150 generates CQI corresponding to the channel estimation result from the channel estimator 165, and transmits the generated CQI to the base station transmitter 100 through a CQI transmitter (not illustrated in FIG. 1).
The symbol demapper 173 demodulates the channel-demultiplexed signal output from the channel demultiplexer 169 using a corresponding demodulation scheme under the control of the resource assignment controller 171, and outputs the demodulated signal to the decoder 175. The decoder 175 decodes the demodulated signal output from the symbol demapper 173 using a corresponding decoding scheme under the control of the resource assignment controller 171, and outputs the decoded signal to the CRC remover 177. The resource assignment controller 171 detects modulation and coding schemes, i.e., MCS level, used in the base station transmitter 100, included in the control data transmitted from the base station transmitter 100, and controls the demodulation scheme of the symbol demapper 173 and the decoding scheme of the decoder 175 based on the detected MCS level. The demodulation scheme and the decoding scheme correspond to the modulation scheme and the coding scheme used in the base station transmitter 100. The CRC remover 177 removes CRC bits from the decoded signal output from the decoder 175, and outputs the CRC-removed signal as information data bits transmitted by the transmitter.
In order for a base station transmitter to dynamically assign resources as described above, i.e., in order to dynamically assign channels and assign MCS level and transmission power, a procedure for feeding back CQI from a mobile station receiver to the base station transmitter is required.
FIG. 2 is a diagram schematically illustrating a process of feeding back CQI in a conventional OFDM communication. More specifically, FIG. 2 is a diagram schematically illustrating positions at which pilot signals are transmitted, in a frequency domain of a conventional OFDM communication system.
Referring to FIG. 2, an OFDM symbol in the OFDM communication system includes a plurality of subcarriers. Data or a pilot signal is transmitted through each of the subcarriers constituting an OFDM symbol. The number of subcarriers constituting the OFDM symbol can be variably set according to situations of the OFDM communication system. As illustrated in FIG. 2, pilot signals are transmitted through subcarriers in predetermined positions among the subcarriers constituting the OFDM symbol. Black-colored subcarriers represent subcarriers through which pilot signals are transmitted. Herein, a subcarrier through which a pilot signal is transmitted will be referred to as a ‘pilot subcarrier,’ and a subcarrier through which data is transmitted will be referred to as a ‘data subcarrier.’
A conventional OFDM communication system is a fixed radio communication system in which mobile stations are fixedly located in specific positions. In the OFDM communication system, or fixed radio communication system, mobile stations determine CQI for each of all subcarriers received from a base station, and feed back the determined CQIs to the base station. There are various types of information that can be used as the CQI, and it will be assumed herein that an SNR is used as the CQI.
Referring to FIG. 2, a transmitter transmits pilot signals through only pilot subcarriers in predetermined positions. A receiver previously knows positions of the pilot subcarriers transmitted by the transmitter, and also knows the pilot signals transmitted through the pilot subcarriers. Here, the pilot signal is a predetermined sequence, and the sequence, i.e., a pilot sequence is prescribed between the transmitter and the receiver. The receiver calculates a channel gain in the pilot subcarrier, after dividing a signal received through the pilot subcarrier by a pilot signal transmitted by the transmitter through the pilot subcarrier, and calculates estimated channel gains of subcarriers except the pilot subcarriers, i.e., data subcarriers, by interpolating the calculated channel gains in the respective pilot subcarriers. Further, the receiver calculates SNRs of the pilot subcarriers and the data subcarriers by dividing the estimated channel gains of the pilot subcarriers and the data subcarriers by noise energy. The calculated CQIs, for example, SNRs for all subcarriers are fed back to the transmitter, for example, a base station and the base station controls a modulation scheme and a coding scheme for corresponding subcarriers using the CQIs for the subcarriers, fed back from the receiver, for example, mobile station. Herein, because the conventional OFDM communication system is a fixed radio communication system, it is assumed that once subcarriers are assigned to the mobile station, a channel condition of the subcarriers assigned to the mobile station is constant.
A description will now be made of a resource assignment scheme in a case in which CQIs for subcarriers are fed back. Here, a resource assignment scheme described below corresponds to a resource assignment scheme of the resource assignment controller 115. Additionally, herein, an OFDM communication system, or a fixed radio communication system, having one base station and a plurality of mobile stations will be taken into consideration. Further, an OFDM symbol vector x received at a mobile station will be defined as x={x1, x2, . . . , xN}. N denotes the total number of subcarriers in the OFDM communication system, and respective parameters of the received OFDM symbol vector x can be expressed as shown in Equation (1).xn=gnPnsn+nn  (1)
In Equation (1), gn denotes a complex channel gain of an nth subcarrier, Pn denotes transmission power assigned in a transmitter, for example, a base station, sn denotes a transmitted data symbol, and nn denotes a mean-0, variance-N0 complex Gaussian noise.
An nth subcarrier γn can be defined as shown in Equation (2).
                              γ          n                ≡                                                                          g                n                                                    2                                N            0                                              (        2        )            
The currently available general resource assignment scheme, i.e., a scheme for assigning channel, MCS level, and transmission power, has been proposed on the assumption that a mobile station feeds back CQI for each of all subcarriers used in the OFDM communication system. Herein, a set of CQIs for the subcarriers constituting the OFDM communication system will be referred to as total CQI.
The resource assignment scheme will now be described herein below. First, it will be assumed that a base station transmits data targeting K users, i.e., K mobile stations. It will also be assumed that the base station has received the total CQIs fed back from the K mobile stations. CQI for an mth subcarrier of a kth mobile station among the K mobile stations will be defined as γ(k)m.
The resource assignment scheme for the case where the total CQI is fed back from the mobile stations has a 2-step algorithm including a first step for channel assignment and a second step for MCS level and transmission power assignment. In addition, an index of a mobile station assigned an mth subcarrier will be defined as km, and transmission power assigned to the mth subcarrier will be defined as Pm. If a channel assignment function based on CQI {γ(k)m: k=1, 2, . . . , K} fed back from the mobile stations is defined as k(•) and a transmission power assignment algorithm is defined as λ(•), the 2-step algorithm for the resource assignment scheme can be expressed as shown in Equations (3) and (4).κm=κ(γm(1),γm(2), . . . ,γm(K)) for m=0,1, . . . ,M−1  (3)Pm=λ(γ1(k1),γ2(k2), . . . ,γM(kK)) for m=0,1, . . . ,M−1  (4)
Accordingly, an SNR profile and mean frequency efficiency based on the SNR profile are uniquely determined according to the 2-step algorithm for the resource assignment scheme.
When the 2-step algorithm for the resource assignment scheme is used, optimal channel and transmission power assignment algorithms for a corresponding mobile station can be expressed as shown in Equation (5) and Equation (6).
                              κ          m                =                                            arg              ⁢                                                          ⁢              max                                      1              ≤              k              ≤              K                                ⁢                                          ⁢                      γ            m                          (              k              )                                                          (        5        )            
In Equation (5), km denotes an index of a mobile station having the maximum channel quality γ(k)m in the case where an mth subcarrier is assigned. Therefore, in the optimal channel assignment algorithm of Equation (5), an optimal channel is assigned to a mobile station having the best channel quality in a corresponding subcarrier.
                                                                                          {                                                            P                      m                                        ,                                          m                      =                      0                                        ,                    1                    ,                    …                    ⁢                                                                                  ,                                          M                      -                      1                                                        }                                =                            ⁢                                                                                                      ⁢                                                                    arg                    ⁢                                                                                  ⁢                    max                                                                              p                      0                                        ,                                          p                      1                                        ,                    …                    ⁢                                                                                  ,                                          p                                              M                        -                        1                                                                                            ⁢                                                      ∑                                          m                      =                      0                                                              M                      -                      1                                                        ⁢                                                            log                      2                                        ⁡                                          (                                              1                        +                                                                              p                            m                                                    ⁢                                                      γ                            m                                                          (                                                              κ                                m                                                            )                                                                                                                          )                                                                                                                              (        6        )            
If channel assignment is completed in accordance with Equation (5), i.e., if a corresponding subcarrier is assigned to a mobile station having the best channel quality in the corresponding subcarrier, then MCS level and transmission power are assigned. Using a Lagrange equation, an optimal power assignment algorithm can be expressed as shown in Equation (7).
                              P          m                =                  {                                                                                          1                    /                                          γ                      0                                                        -                                      1                    /                                          γ                      m                                              (                                                  κ                          m                                                )                                                                                                                                                              for                    ⁢                                                                                  ⁢                                          γ                      m                                              (                                                  κ                          m                                                )                                                                              ≥                                      γ                    0                                                                                                      0                                                                                  for                    ⁢                                                                                  ⁢                                          γ                      m                                              (                                                  κ                          m                                                )                                                                              <                                      γ                    0                                                                                                          (        7        )            
In Equation (7), γ0 satisfies a condition of Equation (8).
                                          ∑                          m              =              0                                      M              -              1                                ⁢                      P            m                          =        MP                            (        8        )            
In Equation (8), MP denotes a total transmission power available in the OFDM communication system. The transmission power assignment algorithm based on Equation (7) and Equation (8) is generally called a water-pouring algorithm. The water-pouring algorithm is an optimal transmission power assignment algorithm in which a transmitter maximizes a data rate for available transmission power when the transmitter knows CQIs of independent channels in a communication system having a plurality of the parallel independent channels. The transmitter assigns transmission power for a corresponding channel, and then determines an MCS level to be applied to the corresponding channel based on the CQI. However, as shown in Equation (5) and Equation (6), optimal channel and transmission power assignment has optimal effects only when CQIs for all subcarriers of the OFDM communication system are fed back with one constant.
However, in the current 4G mobile communication system, it is considered that OFDM scheme is utilized to a mobile communication system. Therefore, it is not preferable to assume that channel conditions of once assigned subcarriers are constant. That is, if subcarriers are assigned to a mobile station, channel conditions continuously change. Therefore, the mobile station must feed back variable CQIs for the subcarriers in order to normally use the resource assignment scheme. However, disadvantageously, an operation of frequently feeding back CQIs for all subcarriers in order to use an OFDM scheme in a mobile communication system causes signaling overhead, and signaling for feeding back CQIs for the subcarriers acts as uplink interference. Accordingly, there is a demand for a scheme for efficiently assigning resources, while minimizing signaling overhead caused by the feedback of CQIs in the OFDM mobile communication system.